Field of the Invention
The exemplary, illustrative, technology herein relates to systems, software, and methods for the management of heat engines—commonly using phase change Rankine-like cycle engines, and more particularly, the management of heat capture, storage, and utilization. As discussed herein, the term Rankine cycle refers to the broad family of cycles where the working fluid is pressurized in a liquid state, perhaps to super or sub-critical pressures and/or perhaps using mixtures of fluids such as those in the Kalina cycle.
The Related Art
Systems that capture and reuse waste heat have been extensively described in the art. Generally, these systems involve a series of engineering tradeoffs to optimize efficiency by optimizing the heat transfer from the heat source to a Rankine engine that converts the transferred heat energy into mechanical energy. Typically, these tradeoffs focused on heat exchanger efficiency and in working fluid characteristics.
Heat exchanger efficiency is a function of the heat exchanger materials and the design of the heat exchanger, while working fluids are optimized to match heat capture and release characteristics of the fluid to the working temperature of the system.
Cost effective energy storage is very important. Electrochemical energy storage has strengths but often have cost, safety, and lifetime concerns. Mechanical energy systems such as pumped hydro and compressed air energy storage provide the vast majority of large scale electricity storage capacity today and have proven long term reliability and acceptable performance.
A suitably efficient and cost effective compressed fluid system needs to store the energy cost effectively and have a thermodynamic process that is efficient in both directions: charging and discharging. A common way this is attempted is for the states of the process to be very similar in each step of the process in each direction; that is, the pressures and temperatures at each point in the process are very similar between the charge process and the discharge process.
There are two broad categories for compressed fluid mechanical energy storage systems: 1) heat pumps as part of a pumped thermal energy storage (PTES) systems and 2) compressed fluid energy storage (CFES) systems, where compressed air energy storage (CAES) systems are a widely explored subset of that area. The distinction here is that the fluid may not be air and it may not always be a gas—it might be a supercritical fluid and there may be parts of the process where the fluid is in liquid phase, or a combination of liquid and gas phases.
PTES systems generally store the energy in a thermal energy difference between some masses. Charging involves increasing the amount of mass that has the temperature (or more rigorously enthalpy) difference, or increasing the enthalpy of a fixed mass [in relation to the environment], or some of both, and discharging is taking that potential energy and converting into to mechanical and/or electrical work.
A classic challenge in trying to use a fluid besides air in mechanical energy storage systems is that it generally requires storage of the low pressure fluid, which is often very voluminous, causing cost and site packaging challenges. Also, if the fluids are chemicals, like refrigerants, the cost of the material has to be considered and as well as the risks and costs of leakage from the system.
The technology herein has applications in the areas of capture and management of waste heat, and the management and optimization of heat engines driven from this heat.